Appl Phys B (2010) 101: 1–5DOI 10.1007/s00340-010-4208-2

R A P I D C O M M U N I C AT I O N

Demonstration of high-speed 1D Raman scattering line imaging

K.N. Gabet · N. Jiang · W.R. Lempert · J.A. Sutton

Received: 7 July 2010 / Revised version: 26 July 2010 / Published online: 11 September 2010© Springer-Verlag 2010

Abstract In this manuscript, we demonstrate high-speed(10-kHz acquisition rate) 1D Raman scattering line imagingunder turbulent flow conditions. Because of the inherentlyweak scattering process, Raman scattering measurementstypically require high laser pulse energies that are not avail-able from commercial high-repetition-rate laser systems.Using a custom pulse-burst laser system at OSU, we demon-strate the ability to generate a series of high-energy laserpulses with 100 µs pulse separations, which can be used forhigh-speed Raman scattering imaging. The ultimate goal is acapability of quantitatively measuring all major combustionspecies and deducing time-varying mixture fraction profilesin turbulent combustion environments. Toward this goal, wereport initial results depicting the Raman scattering imagingof O2, N2, CH4, and H2 in a turbulent non-reacting CH4/H2

jet issuing into air. To the authors’ knowledge, this repre-sents the first reported temporally-sequential 1D image se-quences of major species measured via high-speed Ramanscattering.

1 Introduction

Spontaneous Raman scattering (SRS) is a well-establishedtechnique for the simultaneous measurement of majorspecies concentrations, which, for example, when combinedwith Rayleigh scattering (for temperature measurements)allows the deduction of the mixture fraction (ξ) in non-premixed combustion environments. Single point and line

K.N. Gabet · N. Jiang · W.R. Lempert · J.A. Sutton (�)Department of Mechanical and Aerospace Engineering, The OhioState University, E447 Scott Laboratory, 201 West 19th Avenue,Columbus, OH 43210, USAe-mail: sutton.235@osu.eduFax: +1-614-292-3163

imaging measurements have been paramount in furtheringthe understanding of species mixing and turbulent-chemistryinteractions under combusting conditions (see, e.g., [1–9]).Because turbulent combustion processes are highly tran-sient, species concentration measurements that are resolvedin both space and time are highly desired. However, this re-quirement dictates that the data acquisition of scalar fieldsshould be at a much faster sampling rate than typical char-acteristic time-scales of turbulent processes (�1 kHz).

Recent advances in solid-state, diode-pumped lasersand high-speed camera (e.g., CMOS) technology have al-lowed laser diagnostic approaches such as particle-imagingvelocimetry (PIV) and planar laser-induced fluorescence(PLIF) to be extended into the high-speed (kHz acquisi-tion rate) domain (see, e.g., [10–23]). While high-speed PIVmeasurements require low pulse energies due to high signallevels from the Mie scattering of particles, scalar measure-ments are much more complicated; thus, high-speed imag-ing has been restricted to tracer-LIF measurements undernon-reacting conditions (see, e.g., [10–13, 16, 18, 23]) andOH PLIF under reacting conditions (see, e.g., [14, 15, 17,19–22]). Considering only reacting flows for the moment,high-speed OH PLIF imaging has been very useful in char-acterizing transient events such as ignition and extinction;however, the access to important fundamental scalars suchas temperature and mixture fraction has not been available.

Mixture fraction (ξ) imaging has been an important focusof combustion diagnostics for more than two decades, withseveral approaches using various combinations of laser di-agnostics including Raman scattering, Rayleigh scattering,and laser-induced fluorescence (or variants thereof) to de-duce ξ (see [24] and references within). However, the mea-surement of all of the major species via Raman scatteringshould yield the best accuracy in mixture fraction over abroad range of conditions. While Raman (and/or Rayleigh)

2 K.N. Gabet et al.

scattering diagnostics are common at low-repetition rates,the low pulse energies from commercially-available high-repetition-rate laser systems are prohibitive to Raman (andRayleigh) scattering diagnostics. The application of theseimaging techniques in a high-speed or temporally-sequentialmanner would represent a significant step forward towardunderstanding the physical processes governing turbulentflow and flame dynamics. In this paper, we present the recentsuccess in our laboratory using pulse-burst laser technology(see, e.g., [25–28]) to produce a series of high-energy laserpulses that are used for high-speed 1D Raman scattering ina turbulent non-reacting CH4/H2 jet issuing into air.

2 Experiment

In this paper, we consider a turbulent isothermal jet issu-ing from a circular tube into an annular, low-speed (0.3m/s), co-flowing stream of air. The jet fluid, which is amixture of 40% CH4 and 60% H2 by volume, issues fromthe fuel tube at 25 m/s corresponding to a Reynolds num-ber of 9,000 based on tube diameter. The jet-fluid mixturewas chosen so that the mixture-averaged Rayleigh scatter-ing cross-section was constant across pure jet fluid, pure air,and any mixture of the two streams. In this way, a simulta-neous Rayleigh scattering measurement1 allows for a simplecorrection for pulse-to-pulse energy fluctuations under theassumption of negligible differential diffusion effects. How-ever, it should be noted that differential diffusion effects thatmay arise from hydrocarbon/H2 mixtures are explicitly ac-counted for under combusting conditions since the temper-ature and major species are solved simultaneously using aniterative process (see, e.g., [1–9]). Measurements are per-formed at x/D = 10 and r/D = 1, where x is the axial po-sition, r is the radial position, and D is the tube diameter.

2.1 Pulse-burst laser system

The pulse-burst laser system at Ohio State, shown schemat-ically in Fig. 1a, has been described in detail previously inRef. [26] and thus will only be described briefly here. Thelaser system is a master oscillator, power amplifier (MOPA)design, which consists of a single-frequency (<10−3 cm−1)

cw diode-pumped ring laser operating at 1064 nm servingas the primary oscillator, an electro-optic dual Pockels cell

1Simultaneous Rayleigh/Raman scattering measurement capabilitiesare integrated into the collection system as discussed in Sect. 2.2. Un-der the non-reacting conditions currently reported, the Rayleigh scat-tering signal is simply used as a pulse-energy monitor since the numberdensity and Rayleigh scattering cross-sections are constant throughoutthe flowfield; while under combustion conditions, the Rayleigh scatter-ing channel provides temperature information and an additional exter-nal pulse energy monitor is added to the system.

Fig. 1 (a) Schematic diagram and optical layout of pulse-burstNd:YAG laser at OSU. (b) A typical ten-pulse laser burst trace at532 nm with 100 µs inter-pulse spacing (10-kHz acquisition rate).Pulse-to-pulse intensity variations are less than 8%

pulse slicer, and a series of flashlamp-pumped Nd:YAG am-plifiers. The cw laser is initially pre-amplified in a double-pass variable pulse width (0.3–2.0 ms) flashlamp-pumpedamplifier and then formed into a “burst” of laser pulses byrapidly rotating the polarization of the pre-amplified pulseby one of two Pockels cells as described by Wu et al. [25].In the present experiment, the slicing process creates atrain of 15-ns wide pulses, which are separated by 100 µs,corresponding to a repetition rate of 10 kHz. The pulsetrain is then further amplified by a series of four additionalflashlamp-pumped Nd:YAG amplifiers, resulting in a systemgain of ∼3 × 108.

To reduce amplified spontaneous emission (ASE) buildupin the system, a phase conjugate mirror (PCM) is placed be-tween amplification stages 3 and 4 [26]. The PCM is anoptical cell filled with a high index-of refraction liquid (e.g.,FC-75 or CS2) that uses the principle of stimulated Bril-louin scattering (SBS) to act as an intensity filter and breakthe unwanted ASE growth. In addition, the SBS PCM elim-inates the low-intensity pedestal which is superimposed on

Demonstration of high-speed 1D Raman scattering line imaging 3

the desired pulse-burst sequence due to the finite on/off con-trast ratio of the Pockels cell pulse slicer. If the pump beamintensity is above a minimum threshold, a coherent beamis backscattered at 180° and the desired high-intensity laserburst is backscattered toward the final amplifier stages, whilethe sources of low-intensity background (e.g., ASE) do notexceed minimum threshold and pass through the PCM cell.Finally, the series of 1064-nm laser pulses are frequency-doubled to 532 nm with a KD*P crystal. In the current work,the individual energies of the 532-nm pulses are limited to150 mJ to avoid dielectric breakdown of the gases in theprobe volume. Future work will include a three-leg pulsestretcher so that higher laser pulse energies can be used. Anexample series of 532-nm pulses are shown in Fig. 1(b),where the intensity difference between the pulses is lessthan 8%.

2.2 Raman (and Rayleigh) scattering line imaging system

We have assembled a Raman line imaging system similar tothat at Sandia National Laboratories (see, e.g., [1–5, 7, 8])and the German Aerospace Center (DLR) (see, e.g., [3, 9]),with a few notable exceptions, that combines Rayleigh andRaman scattering to obtain high-speed measurements oftemperature and major species in turbulent reacting flows.As a first step, we demonstrate the ability to acquire 10-kHz Raman scattering image sequences in an isothermalturbulent jet. Referring to Fig. 2(a) as a reference, the 532-nm output from the pulse-burst laser is focused through a750-mm focal length plano-convex lens to a spot size ofapproximately 180 µm. A pair of 150-mm diameter achro-mats (Qioptic Linos Photonics) collects both Raman andRayleigh scattered light, which is focused through an ad-justable slit of a custom-built high-throughput transmissionimaging spectrometer, similar to that described in Ref. [29].One high-speed CMOS camera (Vision Research, Phantom710) for the Rayleigh scattering and one intensified (LaV-ision HS-IRO) CMOS camera (Vision Research, Phantom710) are integrated into the imaging spectrometer.

As shown in Fig. 2(b), the optical system consists offive commercially-available camera lenses, wavelength fil-tering optics, and one custom transmission holographicgrating (Kaiser Optical) to disperse the species-specificRaman-shifted light. Upon entering into the spectrometer,the Rayleigh- scattered (532-nm) portion of the collectedlight is split off using a dichroic beamsplitter and focusedonto one of the CMOS cameras for 1D Rayleigh imagingwith a magnification of 1.2. Again, in the present study, theRayleigh signal is simply used to correct for laser pulse-to-pulse variations. The remaining light (550–700 nm) istransmitted through the optical train of the Raman spec-trometer. The four camera lenses are specifically matchedto the desired magnification ratio and spectral dispersion of

Fig. 2 (a) Overview of experimental setup for high-speed 1D Ra-man/Rayleigh scattering imaging. (b) Detailed schematic and opticallayout of the high-throughput, imaging spectrometer, which integratesthe cameras for both Raman and Rayleigh scattering into one unit

the system. The four lenses used in conjunction with theRaman-portion of the optical system are all Nikon cam-era lenses consisting of 85-mm f/1.4, 50-mm f/1.2, 85-mmf/1.4, and 105-mm f/1.8 lenses. This combination of lenses(when coupled to the 150-mm diameter f/4 and f/2 collectionachromats) results in an overall system magnification of 1.5for the Raman scattering. The Raman spectrum is dispersedthrough the custom transmission grating (1200 lines/mm,22° incident/refracted angles, >85% efficiency from 550 to680 nm) and onto one intensified-CMOS camera.

3 Sample results and discussion

The demonstration of high-speed 1D Raman scatteringimaging is conducted under very favorable conditions, that

4 K.N. Gabet et al.

Fig. 3 10-kHz sequence of 1DRaman scattering images in aturbulent CH4/H2 jet issuinginto a co-flowing stream of air.Images are centered at x/d = 10and r/d = 1

is, under isothermal, non-reacting flow conditions. Thesetest conditions represent those of higher signal strength andwill serve as a benchmark test case for future experimentswith signal collection enhancements. Figure 3 shows a ten-image sequence obtained at a 10-kHz acquisition rate of themajor species profiles (O2, N2, CH4, and H2) in the turbu-lent CH4/H2 jet issuing into air. Image processing consistsof darkfield and background image subtraction, correctionsfor camera/intensifier spectral response, and normalizationby laser energy. Since quantitative species measurementsare not obtained in the present demonstration, correctionssuch as non-uniform sensor (pixel) response were not per-formed. Each image corresponds to a field-of-view of 7 mm,which currently is limited by the width of the entrance slitto the spectrometer. However, the smaller field-of-view isbeneficial when using a two-stage image intensifier (as inthe present study) as the use of image intensifiers can resultin artificial gradient broadening in regions of large imagegradients due to local MTF and blurring effects. For smallfield-of-views, the physical gradients of the species concen-trations are spread out over larger portions of the sensor andsubsequently are not as adversely affected as would be thecase of larger field-of-views with larger gradients with re-spect to finite pixel spacing. In order to increase the signal-to-noise ratio (SNR), the images were smoothed with a 13pixel median filter along the spatial dimension of the CMOSsensor. This results in an in-plane spatial resolution of ap-proximately 170 µm, which is matched to the spatial reso-lution defined by the focused laser beam. It should be notedthat by simple median filtering, we can reduce the “shot”noise imposed by the intensifier, but we cannot reduce read-out noise, as the current CMOS sensors cannot be binnedon chip. Future work will include investigating alternativenoise-suppression methodologies that are appropriate for thehigh-speed imaging.

The images in Fig. 3 were broken into seven columns cor-responding to the rotational-vibrational Raman bands of themajor species (O2, N2, CH4, and H2) and potential back-ground interferences at selected spectral regions (negligibleunder non-reacting conditions). Each column (nominally 15pixels wide) was integrated in the spectral dimension of thesensor; the integrated columns corresponding to background

Fig. 4 Example radial profiles (from the images of Fig. 3) of the Ra-man scattering of CH4 as a function of time. Radial distance (in mm)is measured from the jet centerline. Every other radial profile is plottedin red for viewing clarity

interference was subtracted from those corresponding to ma-jor species profiles; and the resulting radial profiles of O2,N2, CH4, and H2 were determined as a function of time. Anexample of one of these sets of profiles is plotted for CH4

(in arbitrary units) in Fig. 4, where only the first 600 µs havebeen shown for clarity. To estimate the SNR of the currentmeasurements, we consider regions of nearly constant airfor O2 and N2 and regions in the fuel core for CH4 and H2.From these image sequences, we calculate the SNR of O2,N2, CH4, and H2 as 6, 11, 30, and 5, respectively. The SNRvalues for O2 and N2 agree with those made in still air undernearly identical laser and signal collection conditions. How-ever, it is clear that the SNR does not scale as the squareroot of the signal (as would be expected under “shot-noise”-limited systems), indicating that other sources such as theread noise of the HS-IRO/CMOS system is significant forthis lower level of laser energy.

While the SNR of the presented images are modest at thispoint, it should be pointed out that the images were acquired

Demonstration of high-speed 1D Raman scattering line imaging 5

with only 150 mJ/pulse. As noted within the literature, con-ventional low repetition-rate Raman scattering imaging isperformed with >1 Joule/pulse (see, e.g., [1–9]). We arecurrently building a next-generation burst-mode laser sys-tem that is designed for both increases in the laser out-put pulse energy (>1 J/pulse) and burst duration (∼250pulses/sequential images). Combining this increase in laserpulse energy with a new “red-enhanced” high-speed intensi-fier now available within our laboratory, we expect increasesof signal levels by a factor of 15–20 and SNR by a factor of>4.

Acknowledgements The support of Air Force Office of ScientificResearch grant FA9550-09-1-0272 (Julian Tishkoff—Technical Mon-itor) is greatly appreciated. The authors acknowledge previous fi-nancial support for the development of the pulse-burst laser sys-tem from NASA (Paul Danehy—Technical Monitor), the U.S. AirForce Research Laboratory—Propulsion Directorate (James Gord—Technical Monitor), and the Air Force Office of Scientific Research (J.Schmisseur—program monitor). The authors also wish to thank R.S.Barlow from Sandia National Laboratories for very useful discussionsconcerning the Raman scattering imaging system.

References

1. J.H. Frank, R.S. Barlow, Proc. Combust. Inst. 27, 759 (1998)2. R.S. Barlow, J.H. Frank, Proc. Combust. Inst. 27, 1087 (1998)3. W. Meier, R.S. Barlow, Y.L. Chen, J.Y. Chen, Combust. Flame

123, 326 (2000)4. A.N. Karpetis, R.S. Barlow, Proc. Combust. Inst. 29, 1929 (2002)5. R.S. Barlow, A.N. Karpetis, Proc. Combust. Inst. 30, 673 (2005)6. D. Geyer, A. Kempf, A. Dreizler, J. Janicka, Proc. Combust. Inst.

30, 681 (2005)

7. R.S. Barlow, A.N. Karpetis, Flow Turbul. Combust. 72, 427(2005)

8. G. Wang, A.N. Karpetis, R.S. Barlow, Combust. Flame 148, 62(2007)

9. L. Wehr, W. Meier, P. Kutne, C. Hassa, Proc. Combust. Inst. 31,3099 (2007)

10. J.D. Smith, V. Sick, Appl. Phys. B 81, 579 (2005)11. J.D. Smith, V. Sick, Appl. Opt. 44, 6682 (2005)12. C.M. Fajardo, J.D. Smith, V. Sick, Appl. Phys. B 85, 25 (2006)13. C.M. Fajardo, V. Sick, Proc. Combust. Inst. 31, 3023 (2007)14. C. Kittler, A. Dreizler, Appl. Phys. B 89, 163 (2007)15. W. Paa, W. Muller, M. Stafast, W. Treibel, Appl. Phys. B 86, 1

(2007)16. C.M. Fajardo, V. Sick, Exp. Fluids 46, 43 (2009)17. M.E. Cundy, V. Sick, Appl. Phys. B 96, 241 (2009)18. B. Peterson, V. Sick, Appl. Phys. B 97, 887 (2009)19. I. Boxx, C. Heeger, R. Gordon, B. Bohm, A. Dreizler, W. Meier,

Combust. Flame 156, 269 (2009)20. I. Boxx, M. Stohr, C. Carter, W. Meier, Appl. Phys. B 95, 23

(2009)21. B. Bohm, C. Heeger, I. Boxx, W. Meier, A. Dreizler, Proc. Com-

bust. Inst. 32, 1647 (2009)22. I. Boxx, C. Heeger, R. Gordon, B. Bohm, M. Aigner, A. Dreizler,

W. Meier, Proc. Combust. Inst. 32, 905 (2009)23. R.L. Gordon, C. Heeger, A. Dreizler, Appl. Phys. B 96, 745

(2009)24. R.S. Barlow, Proc. Combust. Inst. 31, 49 (2007)25. P. Wu, W.R. Lempert, R.B. Miles, AIAA J. 38, 672 (2000)26. B.S. Thurow, N.B. Jiang, M. Samimy, W.R. Lempert, Appl. Opt.

43, 5064 (2005)27. J.D. Miller, M. Slipchenko, T.R. Meyer, N.B. Jiang, W.R. Lem-

pert, J.R. Gord, Opt. Lett. 34, 1309 (2009)28. N. Jiang, M. Webster, W.R. Lempert, Appl. Opt. 48, B23 (2009)29. R.S. Barlow, G.-H. Wang, P. Anselmo-Filho, M.S. Sweeney, S.

Hochgreb, Proc. Combust. Inst. 32, 945 (2009)